1. Field of the Invention
The present invention relates to an accumulator fuel injection system.
2. Description of Related Art
An accumulator fuel injection system (a common rail system) is known for use as a fuel injection system in an internal combustion engine such as a diesel engine. This accumulator fuel injection system supplies high-pressure fuel, which is accumulated in an accumulation chamber, to each cylinder of the engine to thereby improve an engine performance in a wide operating region from a low speed region to a high speed region. If, however, a fuel injection rate is excessively high just after the start of fuel injection, the use of this fuel injection system would result in an abrupt explosive combustion at the initial stage of combustion. This increases an engine noise and nitrogen oxide (NOx) in exhaust gas.
To solve this problem, an accumulator fuel injection system has been proposed which injects fuel at a lower fuel injection rate at the initial stage of each fuel injection cycle than at the intermediate and later stages, and controls the injection rate according to the operating state of the engine.
Such a fuel injection system is disclosed in, for example, Japanese Patent Provisional Publication No. 8-218967. This fuel injection system controls the fuel injection rate in such a manner as to achieve a low fuel injection rate at the initial stage. This fuel injection system has an injection rate (hereinafter referred to as xe2x80x9cdelta injection ratexe2x80x9d) at which an injection volume starts increasing gradually just after the start of the fuel injection if the engine is operated at a low speed and with a low load. More specifically, this fuel injection system is constructed in such a manner that a first electromagnetic valve is provided in a fuel passage, which connects a common rail as a high-pressure fuel accumulation chamber with a fuel storage chamber of a fuel injection valve, and that a second electromagnetic valve is provided in a passage, which is branched from the fuel passage and reaches a control chamber. The second electromagnetic valve controls the switching of the fuel injection valve. To inject the fuel at a delta injection rate shown in FIG. 2 of the above-mentioned Publication No. 8-218967, the fuel injection system turns off the second electromagnetic valve to raise the back pressure of the control chamber and turns on the first electromagnetic valve in the state where the fuel injection valve is opened, thus discharging the high-pressure fuel in the fuel passage to the lower pressure side such as the fuel tank.
Then, the fuel injection system turns on the second electromagnetic valve to lower the back pressure of the control chamber and make the fuel injection valve openable, and then turns off the first electromagnetic valve to supply the high-pressure fuel to the fuel passage from a high-pressure accumulation chamber. This gradually raises the inner pressure of the fuel storage chamber of the fuel injection valve from low pressure to high pressure. More specifically, by using a response delay period in the increase in oil pressure in the fuel passage, a nozzle needle is lifted to gradually increase the fuel injection rage in order to achieve the delta injection rate when the inner pressure of the fuel storage chamber exceeds the valve opening pressure of the fuel injection valve.
When the engine is operated at a high speed and with a high load, the injection rate is an injection rate at which the injection amount starts increasing sharply to inject a large amount of fuel in a short period of time just after the start of the fuel injection (hereinafter referred to as xe2x80x9crectangular injection ratexe2x80x9d). More specifically this fuel injection system is capable of switching the injection rate between the delta injection rate and the rectangular injection rate according to the operating state of the engine.
International Publication No. WO98/09068 also discloses this kind of fuel injection system. As shown in FIG. 6 of this publication, two accumulation chambers with low pressure and high pressure are provided to execute a pilot injection wherein a low-pressure injection is performed before a high-pressure injection, and an injection wherein a high-pressure injection follows a low-pressure initial injection. In this fuel injection system, a first two-way electromagnetic valve is provided at the downstream side of the high-pressure accumulation chamber and a check valve is provided at the downstream side of the first two-way electromagnetic valve to thereby prevent the change in pressure inside a fuel chamber (fuel storage) of a fuel injection valve and the turbulence of an injection waveform. In this fuel injection system, the high-pressure fuel remaining in the fuel passage after the fuel injection flows into the fuel pressure through the orifice and is regulated to be predetermined pressure by an attached pressure regulator without the necessity of providing an injection pump for the low-pressure accumulation chamber Moreover, in this fuel injection system, the fuel is supplied from the low-pressure accumulation chamber to the fuel passage through a check valve disposed in parallel with the orifice when the fuel pressure in the fuel passage is lowered.
In the former fuel injection system, an injection start timing of the fuel injection valve uses a response delay in the increase in oil pressure in the fuel passage after the second electromagnetic valve is turned on to lower the fuel pressure. Therefore, the injection timing is inaccurate, and there is only a low degree of freedom in the control of the injection rate at the start of the injection since the pressure at the initial stage of the injection is determined according to the injection-valve opening pressure. It is therefore impossible to achieve the optimum fuel injection rate according to the operating state of the engine.
It is therefore impossible to make the best use of the original merits of the accumulator fuel injection system.
The latter fuel injection system is provided with a second electromagnetic valve that controls the injection of fuel from the fuel injection valve. The second two-way electromagnetic valve is opened prior to the opening of the first two-way electromagnetic valve during the pilot injection wherein the low-pressure injection is performed before the high-pressure injection or during the injection wherein the high-pressure injection follows the low-pressure initial injection. During the pilot injection, the second two-way electromagnetic valve is opened for a predetermined period of time and is then closed, and thereafter, the first and second two-way electromagnetic valves are opened at the same time.
Since the pressure at the initial stage of the injection is determined according to the fuel pressure in the low-pressure accumulation chamber in this fuel injection system, there is only a low degree of freedom in the control of the injection rate at the start of the injection. It is therefore impossible to achieve the optimum initial fuel injection rate according to the operating state of the engine.
Accordingly, it is an object of the present invention to provide an accumulator fuel injection system, which is capable of controlling the injection rate according to the operating state of the engine and simplifies the structure.
To accomplish the above object, high-pressure fuel pressurized by a fuel supply pump is stored in a first accumulation chamber, and is supplied to a fuel injection valve provided in an internal combustion engine through a first electromagnetic valve device and a fuel passage. Further, the high-pressure fuel pressurized by the fuel supply pump is supplied to a branch passage, which is connected with the downstream side of the first electromagnetic valve device in the fuel passage, and is stored at lower constant pressure than fuel pressure in the first accumulation chamber.
The first electromagnetic valve device switches the fuel passage between a connected state and a disconnected state. A second electromagnetic valve device is provided in a fuel return passage connecting the fuel injection valve with a fuel tank, and switches fuel, injected from the fuel injection valve, between an injected state and an uninjected state.
When the fuel is injected, the first electromagnetic valve device is opened prior to the opening of the second electromagnetic valve device. This achieves a delta injection rate and a rectangular injection rate with a high degree of freedom.
In one embodiment of the present invention, a period since the first electromagnetic valve device is opened until the second electromagnetic valve device is opened is controlled according to the operating state of an engine. This controls the injection rate effectively for the decrease in exhaust gas and the improvement of fuel economy.
Moreover, the period is controlled to be short when the engine is operated at a low speed and with a low load, and the period is controlled to be long when the engine is operated at a high speed and with a high load. This acquires the optimum injection rate according to the operating state of the engine.
Furthermore, the fuel supply pump is controlled in such a manner as to make variable fuel pressure in the first accumulation chamber according to the operating state of the engine. This makes variable the injection-valve opening pressure and the maximum injection pressure in the achievement of the delta injection rate and the rectangular injection rage according to the operating state of the engine. The control of the maximum injection pressure according to the operating state of the engine raises the degree of freedom in the control of the injection rate and achieves the optimum fuel injection rate according to the operating state of the engine.
In another mode of the present invention, an orifice and a check valve are arranged in parallel in a branch passage, and the first accumulation chamber and a pressure control valve are also provided in the branch passage. Therefore, just after the opening of the first electromagnetic valve device and the second electromagnetic valve device, the high-pressure fuel in the fuel passage is supplied to a second accumulation chamber through the orifice and is regulated to be lower constant pressure than fuel pressure in the first accumulation chamber. If the second electromagnetic valve device is opened prior to the first electromagnetic valve device or if the fuel pressure in the fuel passage is lower than the fuel pressure in the second accumulation chamber regulated to be the constant pressure, the fuel in the second accumulation chamber is supplied from the branch passage to the fuel passage or to the fuel injection valve via the fuel passage through the check valve.